In some locations, the use of subsurface safety valves is mandated. Historically, this has been in locations such as in the Gulf of Mexico where wells were drilled on the Outer Continental Shelf to fairly shallow depths. The subsurface safety valves used in those applications required control lines which went to the surfaces with the subsurface safety valve generally deployed at depths of about 1,000 ft. The subsurface safety valve was maintained in a closed position by a return spring which was sized to keep a sleeve in the position required for the valve to be closed against the hydrostatic forces in the control line, as well as any pressures in the wellbore surrounding the subsurface safety valve.
As wells began to be drilled more deeply and subsurface safety valves had to be placed further and further below the surface, the force necessary for the return spring to resist these forces became far greater with increasing depth. Thus, one approach that was used previously was simply to increase the pressure rating of the control components, including the wellhead, so that they would be suitable for the pressures expected. Other techniques involved use of pressurized chambers to offset the effect of hydrostatic forces or equalization techniques to neutralize the effects of such hydrostatic forces. U.S. Pat. No. 4,660,646 illustrates the use of pressurized chambers to offset the return spring forces. U.S. Pat. No. 5,415,237 illustrates the use of valving arrangements to obtain the requisite pressure balance on the sliding sleeve so as to minimize the forces required to actuate the sleeve against a much smaller return spring.
These techniques were fairly complex, involving numerous moving parts and seals. While they accomplish the purpose of allowing wellheads of a lower pressure rating to be used, even in applications involving significant depths such as 10,000 ft., their cost was high and the numerous components used made maintenance and upkeep an issue. Accordingly, the apparatus and method of the present invention have been developed to allow a simple way to be able to use low-pressure wellheads, even in applications of fairly deep subsurface safety valves in the order of deeper than 10,000 ft., where the pressure rating on the wellhead can be fairly minimal, such as 5,000 PSI, yet the subsurface safety valve can operate effectively. The device can be installed at or near the surface or adjacent the wellhead to make access and maintenance considerably easier. The simple design dictates that the device is low cost and easy to install. By virtue of fairly minor changes in the configuration of the device, any given degree of amplification that would be needed for current applications and those likely to occur in the future can be achieved.